Technical Field
The disclosure of the present disclosure generally relates to a memory device and the method for fabricating the same, and more particularly to a non-volatile memory (NVM) device and the method for fabricating the same.
Description of the Related Art
Non-volatile memory (NVM) devices which are able to continually store information even when the supply of electricity is removed from the device containing the NVM cells has been widespread adopted by bulk solid state memory applications in portable audiovisual entertainment devices, cell phones or digital cameras etc. Recently, various 3D memory devices, such as a 3D vertical-channel (VC) flash memory device that has a multi-layer stack structure may possess a higher density memory and excellent electrical characteristics, e.g. reliability in data storage and high operating speed, has been provided in order to accommodate the rising demand for superior memory.
A typical method for fabricating a 3D VC NVM device includes steps as follows: A plural of insulating layers and conductive layers alternatively stacked with each other is firstly formed. A plurality of trenches are then formed in the multi-layers stack to divide the multi-layers stack into a plurality of rigid stacks each of which has a plurality of conductive strips formed by the patterned conductive layers. A charge-trapping layer with an ONO composite layers structure (including a silicon oxide layer, a silicon nitride layer and another silicon oxide layer) and a channel layer are formed in sequence on the bottoms and sidewalls of the trenches, so as to define a plurality switches at the intersection points of the conductive strips, the charge-trapping layer and the channel layer. However, merely the switches defined on the middle-levels conductive strips of each rigid stack can serve as memory cells for being connected in series by the channel layer to form a memory cells string. The switch that is defined on the topmost of the rigid stack may serve as a string selection (SSL) switch the memory cells string respectively.
Because, the SSL switch includes a portion of the charge-trapping layer and connected in series with the memory cells by the channel layer, the SSL switch may be charged while the memory cells are subjected to a programing/erasing process, and an undesirable threshold-voltage shift of the SSL switch may occur. In order to remain the threshold-voltage of the SSL switch stable, addition control circuitry is thus needed to manipulate voltages applied to the SSL switch to compensate the undesirable threshold-voltage shift. As a result, the power consumption of the 3D VC NVM device may be increased and its operation efficiency may be declined.
Therefore, there is a need of providing a memory device and the method for fabricating the same to obviate the drawbacks encountered from the prior art.